Electrical signaling



une 4, 1929. A, Q Bickn-:SON L7545 n ELECTRICAL SIGNALING I Filed OC.. 50, 1926 Patented une 4, 1929.

UNITED stars' .ALII'ON' C. DICKIESON, OF BROOKLYN,NEW YORK, .ASSIGNOR TO BELL TELEPHONE LABORATORIES INCORPORATED, F NEW YORK, N. Y., A CORPORATION OF NEW YORK.

y ELECTRICAL SIGNALING.

Application filed October 30, 1926. Serial No. 145,169.

This invention relates to electrical sig= naling and particularly to carrier current telephony.

An bject of the invention is to reduce disturbances in carrier current telephone systems.4

A related object of the invention is to i'. 15 intervals in a conversation when no signals are being received. During the actual signaling periods the incoming speech is strong enough tooverpower the noise, but during a pause in the speech the noise continues to be detected and produces an unpleasantv effeet upon the subscribers. ear.

One method of overcoming this difficulty is disclosed and claimed 'in my copending application, Serial No. 145,168 liled October 30, 1926. According to that method, the incoming'side bandis combined in a demod'- ulator with carrier current from a local oscillator which is operating continuously, but when the side band is cut ofi during a pause in the conversation the demodulator is blocked by a large negative grid potential and therefore does not transmit noise to the receiver.

According to a feature of the present invention, the carrier current from the local oscillator, which is necessary to the detection of incoming currents,l is impressed upon the demodulator only when a side band is received, and is cut oi from the demodula-` 'tor during intervals when no side band is received, so that line noise is not admltted to the receiver during the periods between si als. The invention may also be used to a vantage in-the transmitting circuit of a carrier system to control the application of carrier current to the modulator when speech is being transmitted.

The invention will be described in connection with the accompanying drawing in which similar reference characters indicate corresponding parts in the several views, and'in which:

Fig. 1 is a diagrammatic illustration of a.

carrier current receiving circuit embodying the invention;

Fig. 2 is a similar view showing the invention applied to a carrier current' 'transmltting circuit;

Fig. 3 illustrates diagrammatically a'modication of the circuit'shown in Fig. 2, employlng avoice operated relay; and

Fig. 1s a curve illustrating variations OFFICE. d l

in the contact resistance of the voice operated relay shown in Fig. 3.

In Fig. 1, an incoming carrier transmission line 5 terminates in a receiving band filter RBF connected to the receiving circuit 6. The incoming'v currents comprise side band components producedby modulating a carrier current with si naling currents such as-speech variations, t e unmodulated carrier component being suppressed at the distant transmitting terminal, not shown.

The filter RBF selectively transmits currents of the frequencies assigned to the re- -ceiving circuit 6 and suppresses currents of all other frequencies which'may be present on the line 5. This iilter, and each of the other ilters shown in the drawin may be of the type disclosed in U. P .atent N o. 1,227,113 to G. A. Campbe1l,`issued May 22, 1917. l f

The receiving circuit 6 is coupled to the input of a balanced wave combining device or demodulator DM by. means of a transformer 7. The demodulator DM comprises a pair of three-electrode space discharge tubes 8 and 9 having divided input and output circuits each including a common branch conductor. The demodulator is provided with the usual sources of electrical energy to properly polarize the grid electrodes, to heat the cathodes, and furnish anode-cathode current, respectively. The output circuits of the tubes 8 and 9 are coupled Yby means of a.. transformer 10 lto a low frequency line 11 which may be an ordinary subscribers telephone line.

The operation of the balanced demodulator is well understood, and will not be described in detail. When carrier current is impressed upon the common branch of the input circuits, for example through trans-` former 12, the incoming side band is combinedwith the carrier current and the low frequency products of demodulation are imressed through the transformer 10 upon the ow frequency line 11. According to the invention, however, the carrier current is supvplied to the demodulator only when the side and is actually being received from the -distant terminal.

The invention provides a branch circuit including an amplifier Ahaving its input circuit connected across the receiving circuit 6 and its output circuit coupled by means of la-transformer 13 to a sensitive response circuit including a three-electrode space discharge tube rectifier 14. The sensitive re onse circuit including the rectifier tube 14 1s of the type disclosed in my copending application Serial No. 145,170 filed October 30, 1926, in which the space discharge path of the tube 14 is included in one arm of a Wheatstone bridge 15. As described in that application, the grid orcontrol electrode of the rectifier tube 14 is given a negative bias of such a value that large changes in the plate resistance of the tube result from minute changes in the negative grid poten-` tial. `The Wheatsone bridge is balanced whenno side band is received in the circuit 6 so that carrier current from the source 16 will'not flow through the transformer 12 at such time.

-When a side bandA is received in the circuit 6 a portion .of the' incomingcurrent asses into the branch circuit and is amplied by the am lifier A and applied to the input circuit of), the tube 14. The potential thus applied to the grid of the tube 14 produces a substantial change in the plate resistance of the tube and unbalances the Wheatstone bridge. Carrier current from the source 16 is therefore impressed through the transformer 12 upon the common branch of the input circuits of the demodulator DM and is combined with the side band in the demodulator to detect the signal. A band lter BF may be included in the output circuit of .the Wheatstone bridge to suppress currents of all frequencies other than the carrier frequency. The amplitude of the carrier current from the source 16 is not proportional to the incoming side band, but the circuit may be so adjusted that for all incoming amplitudes above a low margin the amount of carrier current released is Substantially constant. Increased sensitivyity is obtained by amplifying the side band in the amplifier A before it is applied to the sensitive response circuit.

Fig. 2 illustrates the invention as applied to a carrier telephone transmitting circuit.

The operation of this circuit is substantially the same as that shown in Fig. 1, except that the input currents comprise Speech variations instead lof the side band, and the carrier current is supplied to the Wave com- Dining device or modulator M under the control of the speech currents. A portion of the speech currents impressed `upon the low frequency line 17 is amplified in the amplifier A yand impressed upon the sensitive re.

erated relay 19 having a contact device 20, l

this relay being of the type disclosed in U. S. Patent No. 1,579,887 to E. J. Pratt, issued April 6, 1926. The sensitive response circuit per Se is claimed in my copending application Serial No. 145,171 liled October 30, 1926. The contact device 20 is included in one arm of the Wheatstone bridge 15. The contact resistance characteristics of the relay 19 are illustrated graphically by the curve of Fig. 4, in which the ordinates represent the resistance and the abscissae represent the input voltage. Thecontact resistance characteristics are such that 4the resistance is negligible up to a certain desired low inputv voltage below the voice range, then increases and remains substantially constant over a fairly Wide range of voltages, and beyond the upper limit of the range it again increases rapidly. When no speech currents are received in the low frequency line 17, the relay 19 does not operate and the Wheatstone bridge is balanced so that no carrier current is impressed upon the modulator .M from the oscillator 16. When speech is received in the line 17, however, a part of the speech current is Vimpressed upon the relay 19 via amplifier A. This relay thereupon starts to operate and the contact resistance increases rapidly, unbalancing the `bridge and permitting carrier current to iow from the oscillator 16 to the modulator M.

The invention is capable of other modiiications and adaptations not specifically referred to but included within the scope of the appended claims. t

What is claimed is:

1. In a signaling system, a continuously operable Wave combining device, means for supplying waves to said device, a source of r ing waves to said device, a wave generator effectively disconnected from said device but adapted to sup ly Waves to said device to be combined Wit other Waves impressed on said device, and means responsive to said first mentioned waves to connect said generator effectively to said device.

3. In a carrier current signaling system, a continuously operable Wave combining device, means for supplying modifyin waves to said device, Va source of waves o carrier frequency, means for amplifying a portion of said modifyin Waves, and means responsive to the amp ified waves to cause said f source to supplylwaves of carrier frequency to said device.

4. In a carrier wave signalin system, a wave combining device, means or supplying modif ing waves to said device, a balanced l*bri gc including a source of waves of carrier frequency, and means responsive to said modifying waves for unbalancing said bridge to effectively connect said source of waves to said device.

5. In a carrier current signaling system, a circuit for receiving incoming signal modulated waves, a demodulator in said circuit, a source of Waves of carrier frequency, and means responsive to said incoming Waves to cause said source to supply waves of'carrier frequency to said demodulator.

6. In a carrier current signaling s stem, a vcircuit for receiving incoming signa mod ulated Waves, a demodulator in said circuit, a carrier wave oscillatorefectively disconnected from said demodulator, and means responsive to said incomin Waves for effectively connecting said osci lator to said de' modulator. l

7. In a carrier current signaling system, a circuit for receiving incoming si al modulated waves, a demodulator in said circuit, a source of sustained carrier waves effectively disconnected from said demodulator, and

means responsive to said incomin waves for connecting said source effective y to said demodulator.

8. In a carrier current signaling system a circuit for receiving incomlng signal m0 u lated waves, a demodulator in said cii'cuit, a source of sustained waves of carrier frequency, means for amplifying a portion of said incoming waves, and means responsive to the amplified waves to cause said source to supply waves of carrier frequency to said demodulator.

9. In a carrier current signaling system, a circuit for receiving incoming signal modulated waves, a demodulator in said circuit, a balanced bride including a carrier wave oscillator, an means responsive to said incoming waves for unbalancing said bridge to connect said oscillator effectively to said demodulator.

10. In a carrier current signaling s stem, a circuit for receiving incoming signa modulated waves, a demodulatorin said circuit, a balanced bridge including a source of sustained waves of carrier frequency, and means responsive to said incoming waves for unbalancing said bridge to cause said source to supply Waves of carrier frequency to said demodulator.

11. In a carrier current signaling system, y va clrcuit for recelving .incoming signal modulated waves, a demodulator in said circuit,

a balanced bridge includin a source of sus-- `A L'rloN c. DICKIESON. 

